Multicellularity and Death

Chapter XI of The Sacred Depths of Nature,
by Ursula Goodenough

The Germ/Soma Dichotomy

Many kinds of sexual algae and fungi are single-celled. Each cell/organism is either a haploid male or a haploid female, and each has two options: it can replicate and divide and replicate and divide to generate millions of identical copies (a mitotic clone) of itself, or else it can recognize and fuse with a cell of the opposite sex to produce a diploid zygote. The zygote switches on genetic programs that allow it to form a protective spore coat around itself and go into dormancy. And then, when circumstances are favorable, the spore undergoes meiosis and releases haploid male and female organisms that are pink/blue mixtures of their parents, and these again either cycle mitotically or else mate with one another.

Multicellular eukaryotes evolved from such single-celled creatures at the time of the Cambrian. We have already considered how multicellular organisms produce all manner of specializations by expressing different sets of genes in different sets of cells. Omitted from that account was the important fact that all multicellular organisms are sexual. Indeed, the invention of sex was necessary for multicellularity to evolve.

To understand what this means, we can consider the diploid zygote-the fertilized egg-of a multicellular animal. Whereas the algal zygote has but modest potential-it can form a spore coat and it can undergo meiosis-the animal zygote proceeds to cleave into two cells, and then four and then eight, with each cleavage generating daughter cells that remain together as a developing embryo. And then all of them start to specialize.

As we have said, each cell expresses only a subset of the genes it possesses, a differential that plays itself out in space and time. Let’s focus on one of the cells in an 8-cell embryo, a cell programmed to switch on a certain set of genes. In the 16-cell embryo this cell has given rise to 2 daughter cells, both containing the protein products of these genes, and the products cause a second subset of genes to switch on. In the 32-cell embryo, the products of the second subset initiate a signal-transduction cascade that induces the now-4 daughter cells in the lineage to move together to a new location and, several cleavages later, to move into the interior of the embryo in a process called gastrulation. Following gastrulation, the lineage (now 512 daughter cells) is subject to several fates: 64 of the cells at one end of the embryo activate a set of genes that allow their daughters to differentiate into gut cells; another 8 near the midline activate the program that ultimately generates the heart; and so on.

Early in this process of embryogenesis, certain cells switch on sets of genes that commit them to become germ-line cells-precursors of the egg or sperm cells that are uniquely capable of undergoing meiosis. These migrate into what will become the animal’s gonads, where they remain dormant until sexual maturity and then begin undergoing meiosis to produce haploid gametes.

We can now appreciate the beauty of this arrangement. The dichotomy between the germline cells and the remaining somatic cells effectively parcels out the job of being alive. Transmission of the genome to the next generation is entrusted to the germ line, while negotiating the niche so that the germ cells are successfully transmitted is entrusted to the soma. The germ line is safely sequestered in gonads, nurtured by surrounding tissues, its genomes released only at appropriate times; the somatic cells are the ones that perceive and move and sprout feathers and pump blood and make love.

Mortality and Immortality

One of the fates that is often programmed into a cell lineage during the course of embryogenesis is that those cells should die. Thus, the limbs of a human embryo initially terminate as blunt stubs, after which sets of cells die in order to create separate fingers and toes. And every autumn, in every deciduous tree, the cells at the base of each leaf stem are programmed to die such that the flow of nutrients is cut off and the leaves themselves die.

The more general fate of the soma is that the whole soma dies. If this death is premature, before the germ line has had time to be successfully transmitted to the next generation, we say that that organism was either unfit (an insect incapable of flight) or unlucky (an insect eaten by a bird). But if it happens after the germ line has successfully participated in the production of sons and daughters, then we say that the organism has served its biological purpose. Natural death may occur after only a few days of life, as with some kinds of adult insects, or it may be postponed for hundreds of years and hundreds of attempted procreation cycles, as is the case for some kinds of trees.

Eventually, though, the sequoias die just like the dragonflies. If we don’t die by accident or infection or because of the failure of a particular organ, we die because we just get old. A friend describes her husband’s last two years before his death at the age of ninety-one: “It just got slower and slower, and less and less, and then he stopped being interested in eating, and then in drinking, and then he stopped breathing.”

So is there such a thing as an immortal organism? The answer is yes, but immortal organisms are by definition very limited in complexity. For example, there is no death programmed into the life cycle of a bacterium or an amoeba. For sure, the cells can be killed by boiling or starvation-the individuals are fully mortal-but they don’t have to die. The same is true for the sexual single-celled algae that we grow in my laboratory. The cells need to have sex when they are in the wild-they must form heavy-walled zygotic spores to protect their genomes from freezing and dessication-but under our care they will keep on dividing indefinitely by mitosis as long as we provide them with light and nitrogen salts. By the same token, tumor cells, in scientific terminology, are said to be “immortalized.” They carry somatic mutations in key cell-cycle genes such that they don’t know when to stop dividing, either in our bodies or in the laboratory.

But once you have a life cycle with a germ line and a soma, then immortality is handed over to the germ line. This liberates the soma from any obligation to generate gametes, and allows it to focus instead on strategies for getting the gametes transmitted. And since morphogenesis is the key niche-negotiating strategy of eukaryotes, multicellular eukaryotes, freed of constraints, have generated every complex morphological structure imaginable: wings, gills, eyes, leaves, glands, claws, bark, nostrils, tentacles. All of these parts are highly specialized, and although each cell in each part retains two full copies of the genome, transmission of these somatic genomes to the next generation is not included in the arrangement. The arrangement is that the parts will do their utmost to ensure the transmission, and often the nurture, of the germ line, and then they die.

One of these “parts” is my brain, the locus of my self-awareness. My brain developed with nary a backward look at gene transmission or immortality. The whole point was to make synapses, strengthen them, modulate them, reconfigure them, with countless neurons dying in the process and countless more dying during my lifetime, many as I sit here typing. It is because these cells were not committed to the future that they could specialize and cooperate in the construction of this most extraordinary, and most here-and-now, center of my perception and feelings.

So our brains, and hence our minds, are destined to die with the rest of the soma. And it is here that we arrive at one of the central ironies of human existence. Which is that our sentient brains are uniquely capable of experiencing deep regret and sorrow and fear at the prospect of our own death, yet it was the invention of death, the invention of the germ/soma dichotomy, that made possible the existence of our brains.

Reflections

All religions offer us a way to think about death, usually in the context of some form of immortality. We know about the heaven/hell of Western traditions and the reincarnation cycles of Asian traditions, but in fact the concept of immortality is global. The Bwende, in the Congo, carved icons to the Four Moments of the Sun: Dawn (the beginning of life), Noon (life at its fullest), Sunset (the end of life’s journey), and a Second Dawn (for those who have lived an exemplary life). The Egyptians developed an elaborate Afterlife ruled by King Osiris and inhabited by numerous gods. The Taoists look to Fei-sheng, the ascension to heaven in daylight. The Muslims anticipate resurrection (yaum al-qiyarna) and final judgment (yaum al-din).

Religious naturalism offers two responses to human death. The first is the response to the death of someone loved, or a death that is premature or senseless. These directly ravage our personal fabric of relationship, or activate our empathy and compassion, and we experience unmitigated loss and grief. I was told of a school-age child whose mother was killed in an automobile accident-how weeks later he would go into her clothes closet and bury his face in her dresses so he could smell her smell. I am undone by his savage loss, and outraged by her death, even though these people are strangers to me. Our sorrow at the death of others is a universal human emotion that transcends cultural and religious particularities. Indeed, ape mothers have been observed carrying their dead babies around for several days, suggesting that this form of grieving far antedates our humanness.

And then there is the response to the fact of death itself, and, in particular, to the fact of my own inevitable death. When I wonder what it will feel like to be dead, I tell myself that it will be like before I was born, an understanding that has helped me to cope with my fear of being dead. But what about the fact that I will die? Does death have any meaning?

Well, yes, it does. Sex without death gets you single-celled algae and fungi; sex with a mortal soma gets you the rest of the eukaryotic creatures. Death is the price paid to have trees and clams and birds and grasshoppers, and death is the price paid to have human consciousness, to be aware of all that shimmering awareness and all that love.